Add brand checks (for known types only I guess) to tryImplForWrapper

[domenic]

For example the first lines of the impl for Node.prototype.insertBefore are currently

  insertBefore(newChildImpl, refChildImpl) {
    // TODO branding
    if (!newChildImpl || !(newChildImpl instanceof NodeImpl)) {
      throw new TypeError("First argument to Node.prototype.insertBefore must be a Node");
    }
    if (refChildImpl !== null && !(refChildImpl instanceof NodeImpl)) {
      throw new TypeError("Second argument to Node.prototype.insertBefore must be a Node or null or undefined");
    }

This should not be necessary; webidl2js should generate the type-checks against Node for the first and Node? for the second.

[TimothyGu]

This should have been fixed in #33.